Why do puffer fish puff up?

Dear Dr. Universe: How do puffer fish puff up? Do they fill with water or air? Why do they get so big? Thank you. – Ben, 6, Madison, WI

Dear Ben,

You’re right, a puffer fish can get pretty big. In fact, some of them can even inflate to the size of a balloon or a beachball.

My friend Wes Dowd is an associate professor in the School of Biological Sciences at Washington State University who currently studies mussels and tiny crustaceans called copepods in tidepools. Much of his training was with fish and he has always been curious about life in the ocean.

In fact he said he’s shared your fascination with puffer fish ever since he was a kid himself, fishing for the Northern puffer fish on the waters of the Chesapeake Bay.

He remembers these fish were a lot different than the bass or flounder he usually caught. The yellowish puffer fish had spines around its body, puffy cheeks, and a little beak-like mouth.

One idea scientists have about the pufferfish is its puffiness is a defense against predators, Dowd explained.

If puffer fish are swimming, they gulp water into a special sack near their belly. But if they are frightened while in the air, perhaps while being fished out of the water, they will also gulp in air.

“In the fish world, dinner is often consumed in one bite,” says Dowd. “So making yourself as big around as possible minimizes your chances of being eaten.”

Being able to puff up while also having lots of spines on your body is also a great way to ward off predators.

One idea scientists have about the ancestors of puffer fish is that they coughed, then they evolved a blowing behavior that many species still use today to find and reveal prey buried under the sand. Dowd explained that the muscles and bones of the head involved in these kinds of behaviors later changed to allow the animal to inflate.

Even if a predator is big enough to eat a puffer fish whole, it will be in for a big surprise. Some pufferfish also have another potent way to defend themselves: tetrodotoxin. It blocks the ability for an animal’s nerves to communicate with each other, causing paralysis or death. However, some puffer fish like the ones Dowd caught as a kid aren’t toxic.

Believe it or not, the puffer fish is just one of more than 200,000 known species of animals that live in our oceans—and there are likely millions more we don’t even know about yet. Who knows Ben, maybe one day you’ll discover something about the puffer fish or other kinds of life out there in the ocean.